In both fixed wing and rotary wing (helicopter) aircraft it is common for the pilot to use a variety of position controls, such as sticks, levers, wheels and pedals, to position the control and aerodynamic surfaces of the aircraft, to thereby control the aircraft attitude, altitude, speed and the like. In more complicated systems, the controls may have mechanical connections which are boosted by hydraulic servos, or the like.
In such an aircraft, where there are two control stations, the controls are mechanically interconnected such that in the event that a pilot is unable to fly, the co-pilot is provided full authority. Additionally, it is possible for one pilot to override the other to instantaneously take control in the event of impending contact with an obstacle. Such control is accomplished by physically applying greater force to the mechanical controls.
To overcome certain limitations of mechanical controls, "sidearm" controllers have been developed which provide a single side-arm controller (control stick) which replaces numerous mechanical controls. In a dual piloted aircraft, each control station is provided with such a sidearm controller. An example of a sidearm controller is described in commonly owned U.S. Pat. No. 4,420,808 entitled "Multi-Axis Force Stick, Self-Trimmed Aircraft Flight Control System." The '808 patent discloses a sidearm controller which provides control signals in each of the pitch, roll, yaw and collective/lift axes of a helicopter control system. The stick is essentially a "force" stick in that there is limited displacement of the stick in each axis, and the magnitude of control signals is directly related to the magnitude and direction of force applied to the force stick. Such controllers are typically utilized in a fly-by-wire flight control system.
As opposed to the gross displacement of prior art mechanical systems which make command and control inputs of the pilot apparent to a co-pilot, no obvious co-pilot cues exist for limited displacement sidearm controllers. Furthermore, fly-by-wire systems have the capability of imparting far greater strength or authority to one of the cock-pit control stations, e.g., the pilot control station may be provided with greater authority to override the co-pilot control station. Accordingly, a decision that the pilot station shall be "stronger" can significantly degrade the control response or agility available to the co-pilot station. On the other hand, greater authority given to the co-pilot station can have the adverse effect of magnifying system sensitivity such that inadvertent inputs by the co-pilot will override pilot commands.
One possible solution to determining the priority of control signals between the pilot and the co-pilot is to provide full authority to the pilot whenever the pilot inputs a control input above a threshold magnitude. Therefore no authority will be given to the co-pilot unless the pilot is not inputting a control signal. When the pilot relinquishes control by reducing input below the threshold magnitude, co-pilot control signals are "faded-in" or "washed-in" to provide aircraft control. In the event of a malfunction of the pilot's control station, a push button switch on the co-pilot grip is provided to de-select the pilot control station and provide the co-pilot control station with instant acquisition of full cockpit authority. For the transition to occur smoothly, the co-pilot control signals may be faded or washed-in as opposed to a step input of a control signal. A problem associated with such a control system is that the co-pilot is unable to input small correcting commands or to "nudge" the aircraft as necessary. Additionally, if the pilot maintains full authority, the co-pilot is unable to react in an emergency situation without de-selecting the pilot control station.
Another proposed solution is to provide the pilot and co-pilot stations with equivalent authority. The signals provided by the pilot and co-pilot stations are summed to provide a total input signal to the control system. A problem associated with this type of control is that if the pilot and co-pilot input opposite control signals, the signals will cancel each other out. This is of particular concern in an obstacle avoidance or combat situation where an emergency maneuver is required. If the co-pilot and pilot react to the emergency situation by commanding opposite inputs, then no control of the aircraft is provided what so ever because the control inputs cancel each other out, therefore placing the aircraft in great jeopardy.